1. Field of the Invention
This invention relates to electroluminescent devices, and more particularly to the fabrication of light emitting polymer devices. Specifically, this invention describes a unique, low cost, encapsulation process that provides a barrier for preventing ambient moisture and oxygen in light emitting polymer devices with air-stable electrodes (cathodes).
2. Description of the Related Art
Typical light emitting polymer (LEP) devices have been under development for flat panel and other display applications for many years. There are other patents that teach how different LEP device layers enable the efficient production of electroluminescent light. For example, in U.S. Pat. No. 5,399,502, Friend et al. teach a method of manufacturing electroluminescent devices, and in U.S. Pat. No. 5,869,350, Heeger et al. teach the fabrication of visible light emitting diodes soluble semiconducting polymers. Including these typical device design, all LEP devices are currently limited in their use due to poor environmental stability. The ingress of water and oxygen into the device creates catastrophic failure mechanisms (e.g., photo-oxidation, electrochemical reactions, metal migration, delamination of the device, etc.) resulting in dark spots, shorting and other failures of the device. These problems, and others, are compounded for LEP devices made on thin flexible plastic substrates because these substrates typically provide little protection against oxygen and moisture ingress. For these reasons all LEP devices of today must be encapsulated between top and bottom barrier layers of some type.
There have been several approaches to solving the problems with LEP devices made on thin flexible plastic substrates. One common technique is to fabricate the LEP device on glass then cover the top of the device by attaching a metal can or a second glass plate with adhesive. The glass and metal of this typical design provides a near hermetic seal. The problem with this technique is that the finished display is ridged, heavy, bulky, fragile and expensive. For LEP devices made on plastic substrates, new technologies have been developed to create transparent, flexible barriers. One example is a series of alternating thin layers of polymer and ceramic materials to create a thin, transparent oxygen and moisture barrier layer that can be applied directly to the device or on to any plastic substrate. Several examples of these polymer-ceramic layered LEP devices can be found in U.S. Pat. No. 6,268,695 to Affinito, U.S. Pat. No. 6,146,225 Sheats et al., U.S. Pat. No. 5,734,225 to Biebuyck et al. and U.S. Pat. No. 6,635,989 to Nilsson et al., for example.
Typical LEP devices require low work function metal cathodes for efficient charge injection into the light emitting polymer layer. The low work function metal cathodes, typically Calcium or Lithium, are not air-stable and quickly oxidize in the presence oxygen. These low work function metals require that the LEP devices never come into contact with air before or during fabrication process, including the encapsulation process. Thus, this requires that the cathodes be deposited in an inert gas environment and then that the LEP device be continually maintained in that inert gas environment until encapsulation is complete. This makes manufacturing difficult, time consuming and expensive.
A solution to this problem of maintaining an inert environment from fabrication through encapsulation has been suggested by Carter et al. in U.S. Patent Application Publication No. 20030153141 filed on Dec. 20, 2002 and entitled “Screen Printable Electrode for Light Emitting Polymer Device.” In this application, Carter et al. describe a method for creating an air-stable electrode (cathode) for an LEP device. Unlike the LEP device with a non-air-stable electrode (cathode), the LEP device with an air-stable electrode (cathode) can be fabricated in an ambient environment and does not need to be maintained in an inert environment throughout fabrication and encapsulation. However, even with an air-stable electrode, moisture and air need to be removed from the device and the device needs to be encapsulated prior to operation because of irreversible electrochemical reactions that can occur in the device in the presence of moisture, air and electric fields.
Therefore, what is needed is a method for encapsulating a pre-fabricated, “off-the-shelf” LEP device having an air-stable electrode (cathode) such that undesirable moisture, air and residual solvents are removed prior to the encapsulation.